Concrete is a composite construction material composed primarily of aggregate, cement, and water. There are many formulations, which provide varied properties. The aggregate is generally a coarse gravel or crushed rocks such as limestone, or granite, along with a fine aggregate such as sand. The cement, commonly Portland cement, and other cementitious materials such as fly ash and slag cement, serve as a binder for the aggregate. Various chemical admixtures are also added to achieve varied properties. Water is mixed with the dry concrete mixture, which enables it to be shaped (typically poured or casted) and then solidified and hardened (cured, set) into rock-hard strength concrete through a chemical process called hydration. The water reacts with the cement, which bonds the other components together, finally creating a robust stone-like material. Concrete can be damaged by many processes, such as the freezing of water trapped in the concrete pores.
Concrete is widely used for making architectural structures, foundations, brick/block walls, pavements, bridges/overpasses, motorways/roads, runways, parking structures, dams, pools/reservoirs, pipes, footings for gates, fences and poles and even boats.
Within the scope of this application, a cementitious composition should be interpreted as comprising anyone of a mortar composition, a concrete composition, and a cement paste composition, which has not been casted, cured, hydrated, set and/or hardened. A mortar composition comprises at least a fine aggregate, such as sand, cement and optionally water. A cement paste composition comprises at least cement and optionally water. A cementitious composition not containing water in such amounts that the setting is initiated is called a cementitious composition in the dry state. According to a preferred embodiment, a cementitious composition is produced by adding all ingredients together and thoroughly mixing said ingredients until a homogeneous composition is obtained.
Within the scope of this application, a cementitious solid should be interpreted as the casted, cured, hydrated, set and/or hardened cementitious composition, comprising anyone of a mortar, a concrete and a cement paste, and water. A cementitious solid is usually obtained by adding water to a cementitious composition, which initiates the curing process. According to a preferred embodiment, the cementitious solid is produced by adding water to the cementitious composition.
Casting and curing concrete in cold weather, in particular at or below a—sustained—freezing temperature is challenging. The most common problem is that concrete freezes and/or goes through freeze/thaw cycles before acquiring adequate strength during curing.
Within the context of this application, “cold weather” is defined when the following conditions exist for at least three consecutive days:
the average daily temperature falls below 4° C., and
the air temperature does not rise above 10° C. for more than half a day in any 24-hour period.
At said cold weather conditions, water starts to freeze in capillaries of concrete at −2° C., it expands up to 9% of its volume when it freezes causing cracks in the concrete matrix, and up to 50% of compressive strength reduction may occur if concrete freezes before reaching at least a compressive strength of 500 psi.
Casting concrete in cold weather follows the recommendations by ACI (American Concrete Institute) Guideline 306R-88. Insulation of the cast concrete, the use of setting accelerators (SA) and of water-reducing agents, also known as superplasticizer (SP), are described as measures to ensure a proper curing of the concrete.
A widely known approach is to add sodium nitrate to the concrete at dosages of up to 5 weight % relative to the concrete composition, comprising at least aggregate, cement, and water. This approach usually delivers a quick-setting cement. U.S. Pat. No. 5,296,028 (Charles J. Korhonen et al., 1994) discloses an antifreeze composition consisting of sodium nitrate and sodium sulphate at a ratio of 3:1, wherein the antifreeze composition is present in the concrete at a dosage of 2 weight % to 8 weight %, relative to the weight of the concrete composition. However, the high alkali addition due to sodium increases the risk of alkali-aggregate-reactions (AAR) and in addition, sodium nitrate is known to significantly reduce compressive strength. Hence, this kind of concrete has a reduced durability, especially when it comes to freeze/thaw-resistance.
Some commercially available products combine several components in one admixture, such as a superplasticizer (SP) and a setting accelerator (SA). Water reduction using a superplasticizer (SP) is a common technique to reduce free water and increase salinity of the pore fluids (which also reduces the freezing point of water). For instance, U.S. Pat. No. 5,176,753 or the equivalent patent GB 2,195,328 (Sandoz, John W. Brook, 1993) describes the combined use of (1) a mineral freezing point depressant, for example calcium nitrate, (2) a superplasticizer, for example the sodium salt of naphthalene sulphonate-formaldehyde resin, (3) an inorganic set accelerator, for example sodium thiocyanate, and (4) an organic set accelerator, for example tetra (N-methylol) glycoluril.
In order to obtain a very quick setting of the concrete, the prior art literature indicates that three-valent ions like aluminium (Al3+) or iron (Fe3+) might be beneficial. This is documented especially for shotcrete (concrete conveyed through a hose and pneumatically projected at high velocity onto a surface, as a construction technique). U.S. Pat. No. 4,444,593 discloses ferric nitrate blends for rapid setting. WO97/36839 (Tjugum, 1997) discloses aluminium-based salts, in particular aluminium nitrate. Shotcrete is not linked to cold weather concreting, as the concrete is, for example, applied in tunnels where no cold weather conditions prevail, in particular no temperatures below the freezing point of water.
Harald Justnes in Concrete, Volume 44, Number 1, February 2010 “Calcium nitrate as a multi-functional concrete admixture”, discloses the use of calcium nitrate as a set accelerator when used with a plasticiser counteracting the retardation by the plasticiser while maintaining rheology, as long-term strength enhancer, in anti-freeze admixtures or winter concreting admixtures, and as a corrosion inhibitor for the protection of embedded steel.
Standards are available describing how to cast concrete that needs to have increased freeze-thaw-resistance, for instance by adding an air-entraining-admixture (AEA).
There is still a need for an admixture that ensures a quick and sufficient hydration of a cementitious composition and improves long term behaviour rather than reducing durability.